Method of effecting solution of lead arsenate



45 i hydrochloric acid being preferred. Such wash- Patented Feb. 15, 1938 PATENT OFFICE METHOD OF EFFECTING SOLUTION OF LEAD ARSENATE Nathan M. Mnookin and Victor E. Speas Kansas City, Mo.

No Drawing.

Application December 13, 1934, Serial No. 757,416

19 Claims. (01. 99-103) The present invention relates to methods of effecting solution of lead arsenate and lead arsenate compounds, for example, in the removal of lead arsenate deposits or residues of sprays from fruits, vegetables and the like, and also in the production of solutions suitable for use in preparing sprays and the like. It will be fully understood from the following description, in which various specific embodiments of the invention are set forth as illustrative thereof.

The common use of lead arsenate alone and in various combinations in horticultural sprays has made important the development of suitable means or methods of efiecting solution of lead arsenate compounds, both in connection with the preparation of sprays and the removal of residues or traces of such spray deposits left on fruits and vegetables. In both cases, it is necessary that the means employed for efiecting solution of the lead arsenate compounds (particularly in the case of removal of such materials from fruits and vegetables) is itself of an innocuous character.

With sprayed fruits and vegetables-it has been very difficult to secure efiective and adequate removal of the small residue or traces of the lead arsenate compounds left as a result of spray deposits. The tolerance limits of the residual lead and arsenic permissible as established by the Department of Agriculture are very low, in the order of 2 parts of lead per million and 1A parts of arsenic trioxide per million on the produce regardless of the character thereof. Where sprayed fruits or vegetables are employed in the production of dried or concentrated products such as in the preparation of dried apples or other fruits, since a concentration of solids or a portion of the solids of the original material takes place and since the tolerance limits for lead and arsenic are the same in the final product, a much more effective removal of lead and arsenic must be efiected.

It has hitherto been the practice to employ dilute acids and dilute alkalis for the removal of traces or residues of lead arsenate on fruitsand vegetables, in general dilute acids such as ing solutions must, however, ordinarily be warmed I tosome extent to secure effective solvent action upon the minute and strongly held traces of lead arsenate on the materials being washed, their solvent capacity is low, they easily become supersaturated and redeposit lead and they are harmful in their action if not themselves thoroughly washed or removed from the fruits or vegetablesv being treated. Furthermore, upon many products, particularly leafy vegetables such as lettuce,

such washing solutions have a deleterious action,

causing loss in freshness, tenderness, flavor or other desirable characteristics.

In accordance with the present invention, it has been found" that an eifective solvent action upon lead arsenate, and' particularly upon the lead arsenate residues on fruits and vegetables may be secured by means of solutions of compounds of the anhydride phosphoric acids, that is, of pyrophosphoric and metaphosphoric acids, and more particularly of the acids and slightly acidified solutions of their salts. For example, to effect removal of traces of lead arsenate remaining on fruits, such as apples, dilute aqueous solutions of pyrophosphoric acid or metaphosphoric acid may be effectively employed. Of the compounds of pyrophosphoric acid and metaphosphoric acids, those of the latter are the more effective and are preferred. While solutions of greater strength may be used, it is preferred that solutions of 0.1 to 1.0% be used. By the use of such solutions, effective removal of final traces of lead arsenate sprays may be secured, the solutions being preferably usd cold since, in the case of the acids, solubility of the lead arsenate residues is not increased and may, in fact, be decreased.

In the use of such solutions, there is apparently a relation between the amount of lead arsenate dissolved and the amount of the effective acid present in the washing solution, which is largely independent of the concentration of the latter. Thus, 1% solutions of metaphosphoric acid, freshly prepared by dissolving the proper quantity of P205 in water, at ordinary temperature (say to F.) dissolve about 1 gram of lead arsenate for each gram of metaphosphoric acid present; similarly prepared aqueous solutions containing 0.5% metaphosphoric acid and 0.2% metaphosphoric acid dissolve substantially the same relative proportions of lead arsenate and a 0.1% solution of metaphosphoric acid dissolves only very slightly less lead arsenate for an equal weight of metaphosphoric acid present. Freshly prepared pyrophosphoric acid acts in a similar manner, although its solvent action is somewhat less; thus solutions of pyrophosphoric acid con- When warmed, say to temperatures above R, such solutions of metaphosphoric and pyrophosphoric acids dissolve somewhat less than the amounts of lead arsenate that they will dissolve at ordinary temperatures or at temperatures up to 170 F.

Such solutions of metaphosphoric and pyrophosphoric acid tend .to lose their effectiveness as solventagents for lead arsenateand lead arsenate residues if permitted to stand for substantial lengths of time; for example, the solvent action of a solution of metaphosphoric acid may decrease in one week's time to about one-third that of the freshly prepared solution. It has been found, in accordance with the present invention, that stable solutions which maintain their effectiveness and which closely approach in effective solvent action the freshly prepared acids, may be prepared by using the salts of metaphosphoric and pyrophosphoric acids, the

solutions of the salts being slightly acidulated.

Thus suitable dilute solutions of the soluble salts of metaphosphoric acid, to which sufiicient acid is added to make the solution acid to methyl orange, have been found to be highly effective solvents for lead arsenate and lead arsenate residues. For this purpose, there have been employed in accordance with the present invention both the soluble inorganic and organic salts of metaphosphoric and pyrophosphoric acids; such as the sodium, potassium, ammonium, and ethyl amine salts. The salt solutions employed are made slightly acid to methyl orange, using any suitable organic or inorganic acid or acid salt for the purpose, such as hydrochloric acid, sulfuric acid, nitric acid, acetic acid, tartaric acid, hydrofluoric acid, sodium acid phosphate, sodium acid sulfate or the like. For washing fruits and vegetables, such salts may be used in acid solution in concentrations of 0.1% to 1% or higher,-

if desired. In general, solutions ranging from 0.25 to 1.0% are employed. Such mildly acidified solutions,like the solutions of the acids, have a solvent action upon lead arsenate and lead arsenate residues that appears to depend largely upon the amount of the metaphosphate or pyrophosphate present. Thus, such an aqueous solution of sodium metaphosphate containing 1% of sodium metaphosphate will dissolve an amount of lead arsenate 0.8 times the amount of sodium metaphosphate present; a 0.5% solution of the sodium metaphosphate prepared in this manner will dissolve an amount of lead arsenate approximately 0.8 times the amount of sodium metaphosphate present and a 0.25% solution thus prepared dissolves an amount of lead approximately 0.7 times the amount of sodium metaphosphate present. Such solutions are substantially stable with respect to their solvent action on lead arsenate and lead arsenate residues, there being but a very slight decrease in solvent action of such solutions on standing a week to a month. With heating, in the case of the acidulated'solutions of the salts, a slight increase in solvent-action on lead arsenate and lead arsenate residues is found.

The action of pyrophosphate salts in solution,

mildly acidulated, is similar to that of the metaphosphate salts, although toa less degree corresponding to the diflz'erence in solubility of lead arsenate and lead arsenate residues in solutions of the acids.

Solutionsprepared in accordance with the present invention, which may be used in concen-' trations of 0.1 to 1% or higher for the washing of fruits and vegetables, are highly efiective in removing final traces of lead arsenate residues therefrom and have been shown to be effective in reducing such residues to such an extent that even dehydrated or concentrated products derived from the materials treated are within the tolerance limits with respect to the presence of lead and arsenic. The solutions are innocuous, do not have any harmful effect upon tender vegetables, do not require use while warm to secure their full effectiveness and not tend to redeposit lead upon the materials being washed.

While the use of dilute solutions for washing purposes has been described, it is readily apparent that stronger and more concentrated solutions may be used for eflecting solution of larger proportions of lead arsenate for use as sprays. Such solutions may alsobe employed in the preparation of alkaline sprays to secure lead arsenate in a state of fine dispersion therein and to secure a finely divided lead arsenate for use in the preparation of sprays. Thus, in the preparation of a spray, a more concentrated solution of lead arsenate in an acid solution of a metaphosphoric or pyrophosphoric acid or salt may be neutralized, if desired in the presence of other spray constituents, thereby precipitating and making available the lead arsenate in finely divided form in the spray material. Or, a finely divided form of lead arsenate may be produced by the neutralization of such solutions of lead arsenate in acid solutions of metaphosphoric ar pyrophosphoric acid or their soluble salts. Lead arsenate in metaphosphoric acid solutions may also be caused to deposit lead arsenate in highly dispersed form by heating the solutions, the solvent action of the metaphosphoric acid being destroyed as it passes over to the ortho form.

We claim:

1. The method of effecting solution of lead arsenateand lead arsenate residues which comprises bringing into contact therewith an acid solution containing an anhydride phosphoric acid compound of the class consisting of metaphosphoric and pyrophosphoric acids and their salts.

2. The method of efiecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith an acid solution of a compound of the class consisting of metaphosphoric acid and its salts.

3. The method of eflecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith an acid solution of a compound of the class consisting of pyrophosphoric acid and its salts.

4. The method of effecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith a solution of metaphosphoric acid.

5. The method of eiiecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith a solution of pyrophosphoric acid.

6. The method of effecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith an acid solution of a metaphosphate.

'7. The method of eifecting solution of lead arsenate and lead arsenate residues which comi prises bringing into contact therewith an acid solution of a pyrophosphate.

9. The method of removing lead arsenate residues from fruits and vegetables which comprises phosphoric and pyrophosphoric acids and their salts.

10. The method of removing lead arsenate residues from fruits and vegetables which comprises washing such fruits and vegetables with a dilute acid solution of a compound of the class consisting of metaphosphoric acid and its salts. 11. The method of removing lead arsenate residues from fruits and vegetables which comprises washing such fruits and vegetables with a dilute acid solution of a compound of the class consisting of pyrophosphoric acid and its salts.

12., The method of removing lead arsenate residues from fruits and vegetables which comprises washing suchfruits and vegetables with an acidulated dilute solution of a metaphosphate salt.

residues from fruits and vegetables which comprises washing such fruits and vegetables with a dilute solution of a compound of the'class consisting of metaphosphoric acid and its salts and containing 0.1 to 1% thereof, said solution being acid to methyl orange.

14. An acid solution of an anhydride phosphoric acid compound of the class consisting of 13. The method of removing lead arsenate metaphosphoric and pyrophosphoric acids and their salts containing dissolvedlead arsenate.

15. An acid solution of a compound of the class consisting of metaphosphoric acid and its salts containing dissolved lead arsenate.

16. An acid solution of a compound of the class consisting of pyrophosphoric and its salts containing dissolved lead arsenate.

17. An acid, solution of a soluble metaphosphate containing lead arsenate.

18. The method of effecting solution oi lead arsenate and lead arsenate residues which comprises bringing into contact therewith an acid solution of a soluble metaphosphate salt which liberates free metaphosphoric acid capable of dissolving lead arsenate and lead arsenate residues in amounts proportional to the quantity of free acid present in the solution.

19. Thegmethod of effecting solution of lead arsenate and lead arsenate residues which comprises bringing into contact therewith an acid solution of soluble sodium metaphosphate which liberates free metaphosphoric acid capable of dissolving lead arsenate and lead arsenate residues in amounts proportional to the quantity of free acid present in the solution.

NATHAN M. MNoomN. VICTOR E. SPEAS. 

